Two common techniques for electronically steering a radio frequency antenna array are phase steering and frequency steering. Generally, phase steered arrays permit relatively large changes in instantaneous frequency without significantly affecting angular accuracy, but typically incorporate a phase shifter or active phase-shift module for each radiating element. In contrast, frequency steered antennas avoid the cost of the phase-shift components, but provide less instantaneous bandwidth at a given scan angle because changes in instantaneous frequency also affect angular accuracy.
For example, some radar applications require greater instantaneous bandwidth, such as to support frequency agile operation and/or FM-chirp pulse compression, than is available from an albeit less complex frequency steered antenna (given the typical sidelobe level requirements). For these applications, a phase steered antenna is the only practical solution to achieve the desired instantaneous bandwidth while maintaining angular accuracy.
The requirement of a phase shifter or active module for each radiating element is a significant cost factor for these antenna systems. By way of illustration, for a phase steered antenna that provides a two-dimensional scan in the X band (around 10 GHz with a wavelength of about 3 centimeters), the typical radiating-element spacing of one-half wavelength requires a radiating element about every 1.5 centimeters.
The conventional approach to reducing the cost of a phase steered antenna array is to thin the array, either by (a) removing phase shifters (sparsely sampling), or (b) by using frequency scan in one dimension. Sparsely sampling only allows removing 5 to 15 percent of the phase-shifters before sidelobe levels are raised significantly. In contrast, using frequency scan in one dimension reduces the number of phase-shifters to one per row or column, or two for monopulse systems (one for each half of the row or column) without significantly impacting sidelobe levels. However, angular accuracy in that dimension degrades due to incremental beam scanning caused by instantaneous changes in frequency, limiting the broadband capability of the antenna.
Accordingly, a need exists for an improved broadband electronically steered antenna array capable of instantaneous bandwidth performance comparable to phase steered antennas, but with significantly fewer phase-shifter components, and without (a) significantly raising sidelobe levels (such as would be caused by sparsely sampling), or (b) significantly degrading angular accuracy (such as would be caused by frequency scanning in one dimension).